Let's take a look at the top five rookies on offense and defense as we enter December.

Offensive Rookie of the Year:

1. Keenan Allen, San Diego Chargers wide receiver: Having earned Philip Rivers' trust as a go-to target by midseason, Allen has been killing defenses with slants and crossing routes. A top 10 receiver over the past five weeks, the third-round draft steal is on pace for a 77/1,124/4 stat line as the best of an impressive rookie wide receiver class. Of all NFL receivers with more than 50 targets, Allen's 74.4 catch percentage leads the league.

3. Giovani Bernard, Cincinnati Bengals running back: Has any running back generated a more spectacular series of touchdowns this season? Bernard's speed, hands and cutting ability have already made him one of the league's most dangerous backs in the passing game. The Bengals would increase their chances of winning by featuring him in the running game as well.

4. Mike Glennon, Tampa Bay Buccaneers quarterback: Glennon completed 62 of 88 passes (70.5 percent) for 785 yards with seven touchdowns and one interception for a 119.7 passer rating en route to Offensive Rookie of the Month honors for November. After becoming the first rookie in NFL history to throw one touchdown pass in his first eight games, his candidacy for top rookie took a hit in Week 13 when the streak screeched to a halt against the Panthers.

5. Zac Stacy, St. Louis Rams running back: The Rams stayed afloat following Sam Bradford's season-ending injury by turning to Stacy as the focal point of their ball-control offense. A complete, three-down back, Stacy is the rare player who makes defenders miss in tight quarters as well as open spaces.

Defensive Rookie of the Year:

1. Tyrann Mathieu, Arizona Cardinals defensive back: What's amazing about Mathieu is that he's already one of the NFL's most versatile and well-rounded defensive backs, playing safety in base packages and switching to cornerback in nickel and dime packages. I haven't seen a single cornerback who can match the third-round rookie in run support or blitzing.

Mathieu has also shown a knack for big plays, punching the ball out of Jared Cook's hands on a would-be long touchdown and tackling Nate Burleson short of the sticks on a potential game-winning drive. The Honey Badger is a key chess piece in Todd Bowles' defense, which rivals Seattle's for the No. 1 spot in Football Outsiders metrics.

2. Kiko Alonso, Buffalo Bills linebacker: After watching Alonso dominate for the first three months of his career, it's hard to believe a player of his obvious athletic gifts lasted to the middle of the second round. Alonso ranks among the inside linebacker leaders in tackles, quarterback hurries, opponents' passer rating and yards allowed per reception. The Bills suddenly have an intriguing defensive nucleus under coordinator Mike Pettine.

5. Eric Reid, San Francisco 49ers safety: Despite an early-season injury to Patrick Willis and a month-long walkabout from Aldon Smith, the 49ers have been the stingiest defense in the league since Week 4. Reid deserves a lot of credit for that turnaround. His range, impact hits and ball skills stand out in a strong rookie safety class. The only concern is that he's already sustained two concussion and was shaken up again on a collision last week.